Switching devices, in particular low voltage switching devices, can be used to switch the current paths between an electrical supply device and loads, and therefore to switch their operating currents. Thus, the switching device opens and closes current paths, allowing the connected loads to be safely connected and disconnected.
An electrical low-voltage switching device, such as for example a contactor, a circuit breaker or a compact starter, has one or more so-called main contacts for switching the conducting paths, which can be controlled by one or also by a number of control magnets or electromagnetic drives, in order to switch the current paths. In principle, in this case, the main contacts include a moving contact bridge and fixed contact pieces, to which the loads and the supply device are connected. In order to close and open the main contacts, an appropriate connection or disconnection signal is passed to the electromagnetic drive, in response to which their armatures act on the moving contact bridges in such a way that the latter carry out a relative movement with respect to the fixed contact pieces, and either close or open the current paths to be switched.
Appropriately designed contact surfaces are provided in order to improve the contact between the contact pieces and the contact bridges at points at which the two meet one another. These contact surfaces are composed of materials such as for example silver alloys, which are applied at these points both to the contact bridge and to the contact pieces, and have a specific thickness.
As a rule, the electromagnetic drive is designed as a solenoid. The solenoid in this case has a plunger coil as excitation coil as well as an armature. For conduction of the magnetic flux, the electromagnetic drive is surrounded by an iron yoke. If a current is now applied to the excitation coil to switch on the switching device, the armature is pulled into the cylindrical opening of the excitation coil. The movement of the armature that finally actuates a contact slider connected mechanically to the armature, which in turn moves the contact bridges to close the main contacts.
A switching device of the kind specified above has a power supply, which generates a low-voltage DC voltage from an alternating input voltage on the network side in the range from 12 V to 24 V for supplying the solenoid current to the excitation coil. Typical input voltages on the network side are 230 V at 50 Hz or 110 V at 60 Hz. Newer clocked power supplies have a broad input voltage range from approximately 100 V to 230 V. The power supply can also supply an electronic control unit and an electronic monitoring unit of the switching device with current.
During the switch-on process, i.e. in the period of time of the connection of the power supply to the excitation coil up to reaching an ON position at which the armature is fully drawn in, the current requirement of the excitation coil is particularly high. This is explained by the magnetizing current for establishing the magnetic field as well as for the conversion of magnetic energy into mechanical kinetic energy. Were this solenoid current to continue to be provided after reaching the ON position, the excitation coil would heat up in such a manner that erosion of the excitation coil and thereby a failure of the switching device would be the result.
For this reason, the solenoid current is restricted to a holding current, which in comparison to the maximum current is substantially smaller during the switch-on process. This can for example be produced by way of a timing circuit which, after a predeterminable time, brings about a limiting of the solenoid current by the power supply. A disadvantage of this solution is that no feedback is obtained about an actual actuation of the electromagnetic drive. It may be that the main contacts of the switching device are not closed at all by the electromechanical drive. This could be the case for example if dirt has accumulated between the armature and the cylindrical opening of the electromagnetic drive, and this therefore results in these two components of the electromagnetic drive being jammed.
As an alternative, the ON position can be interrogated by way of one or more switching contacts, through which the limiting of the solenoid current can then be brought about by the power supply. A disadvantage of this solution is that the contacts of the switches may become dirty. In this case, as in the case described above, the increased solenoid current would then be supplied again by the power supply with the possible negative consequences mentioned above.
Fault sources such as these in particular must, however, be avoided for the safe operation of switching devices, and therefore for protection of the load and of the electrical installation.